Side Impact Injury Risk for Belted Far Side Passenger Vehicle Occupants
Hampton C. Gabler
Kennerly Digges George Washington University
Brian N. Fildes Monash University Accident Research Centre
Laurie Sparke Holden Innovation
In a side impact, the occupants on both the struck, or near side, of the vehicle and the occupants on the opposite, or far side, of the vehicle are at risk of injury. Since model year 1997, all passenger cars in the U.S. have been required to comply with FMVSS No. 214, a safety standard that mandates a minimum level of side crash protection for near side occupants. No such federal safety standard exists for far side occupants. The mechanism of far side injury is believed to be quite different than the injury mechanism for near side injury. Far side impact protection may require the development of different countermeasures than those which are effective for near side impact protection.
This paper evaluates the risk of side crash injury for far side occupants as a basis for developing far side impact injury countermeasures. Based on the analysis of NASS/CDS 1993-2002, this study examines the injury outcome of over 4500 car, light truck, and van occupants subjected to far side impact. The analysis was restricted to 3-point belted occupants. The paper evaluates the risk of far side impact injury as a function of struck body type, collision partner, delta-V, crash direction (PDOF), occupant compartment intrusion, and injury contact source. Injury risk is evaluated using the maximum injury severity for each occupant, by injury severity for each body region, and by Harm, a social cost measure.
The primary objective of both side impact research and side impact regulation to date has been to protect occupants located on the struck side of a passenger vehicle. In a side impact, however, both the occupants of the struck, or near side, of the vehicle as well as occupants on the opposite, or far side, of the vehicle are at risk of injury (Digges and Dalmotas, 2001). The
mechanism of far side impact injury is believed to be quite different than the injury mechanism for near side impact injury. Far side impact protection may require the development of different countermeasures than those which are effective for near side impact protection. In early 2004, an international consortium of universities and crashworthiness research groups, led by the Monash University Accident Research Centre (MUARC), began to examine the problem of far side impact injury risk. The goal of this research program is to investigate far side impact injury to occupants of passenger cars, light trucks and vans. The specific objectives of the project are to establish an improved understanding of the biomechanics of far side impact injury, develop a test procedure for evaluating the potential of injury in a far side impact, and explore new countermeasure approaches for far side impact injury prevention. This paper presents some of the first findings of this project. OBJECTIVE
The goal of this paper is to determine the risk of injury from far side impact crashes in the United States. The specific objectives are to determine the priorities for injury countermeasure development, and to characterize those impact conditions which lead to far side impact injury as a first step toward the development of a far side impact test procedure. APPROACH
The findings presented in this paper were based on the analysis of the National Automotive Sampling System / Crashworthiness Data System (NASS/CDS) from 1993 - 2002. NASS/CDS is a national sample of 4,000 to 5,000 crashes investigated each year by the National Highway Traffic Safety Administration (NHTSA) at up to 27
locations throughout the United States. For a crash to be included in NASS/CDS, at least one of the vehicles in the accident had to be towed from the scene. The analysis was limited to passenger cars, light trucks, and vans subjected to a side impact. For this study, side impact was defined to be a crash in which the general area of damage in the most harmful event was to the left or right side of the car. Any cases in which the vehicle rolled over were excluded. A far side occupant was defined to be either an outboard occupant on the opposite side of a crash or a center seated occupant. For impacts to the driver side of the car, for example, a right front seat occupant would be considered to be on the far side of the car. Likewise, for impacts to the right side of the car, the driver would be considered to be the far side occupant. Only occupants that were restrained by a three-point safety belt were included in this study. MEASURING SOCIAL COST WITH HARM
Harm is one of several methods of measuring the social cost of traffic accidents. Two other more common measures are number of fatalities and number of injuries. Both fatality and injury counts however provide unrealistic snapshots of social cost. Fatal accidents are extremely rare, and unrepresentative of the majority of traffic accidents. Determining research priorities based upon fatal accidents can bias a study to consider only the most catastrophic accident modes at the expense of potentially more prevalent accident modes which are disabling but non-fatal. On the other hand, basing research priorities upon total number of injuries ignores the fact that most injuries are minor abrasions and bruises, and present no significant threat to life.
Recognizing the need for a social cost metric that balanced the number of injuries with the severity of the injuries, Malliaris et al (1982) developed the Harm metric. The Harm metric determines social cost based upon injury severity. Severity is measured using the Abbreviated Injury Scale (AIS) which describes the relative threat to life of an injury (AAAM, 1990). AIS levels range from 0 for no injury to 6 for unsurvivable, or fatal, injuries. The social cost includes both medical costs and indirect costs such as loss of wages. When a person suffers multiple injuries, the Malliaris Harm metric bases the social cost upon the body region with the maximum AIS level. This method can underestimate the overall AIS level as multiple injuries aggravate the total threat to a crash victims life. Fildes et al (1994) developed an enhanced Harm metric, presented below and used in this study, which more correctly accounts for the social cost of persons with multiple injuries.
This method assigns a social cost to each injury, and sums these costs to estimate a total social cost of injury. In this study, Costi, the social cost of an injury i as defined by Fildes et al (1994) was used as a measure of social cost. Costi is a function of the injury severity as measured by the AIS scale, and the body region which has been injured. The cost components include not only treatment and rehabilitation costs but also all other costs to society such as loss or wages and productivity, medical and emergency service infrastructure costs, legal and insurance costs, legal and insurance charges, family and associated losses and allowances for pain and suffering.
Table 1. Average Cost per Injury (Normalized to the Cost of a Fatal Injury)
INJURY SEVERITY BODY Minor Moderat
e Serious Severe Critical Maximum Unknow
n REGION (AIS = 1) (AIS = 2) (AIS =3) (AIS = 4) (AIS = 5) (AIS = 6) External 0.0045 0.0250 0.0698 0.1135 0.1646 1.0000 0.0045 Head 0.0063 0.0295 0.1213 0.2796 0.9877 1.0000 0.0045 Face 0.0063 0.0295 0.1213 0.1601 0.3277 1.0000 0.0045 Neck 0.0063 0.0295 0.1213 0.1601 0.3277 1.0000 0.0045 Chest 0.0045 0.0250 0.0698 0.1135 0.1646 1.0000 0.0045 Abdomen 0.0045 0.0250 0.0698 0.1135 0.1646 1.0000 0.0045 Pelvis 0.0045 0.0250 0.0698 0.1135 0.1646 1.0000 0.0045 Spine 0.0045 0.0250 0.1631 1.4054 1.6804 1.0000 0.0045 Upper Extremity
0.0063 0.0433 0.1026 0.0045
0.0045 0.0433 0.1303 0.1926 0.3277 0.0045
This study uses a variation of the Fildes method for computation of Harm. In some cases, there may be multiple injuries to a single body region. In our methodology, the maximum injury to a single body region is used when assigning costs as costs are typically assigned to treat a single body region not individual injuries of that body region. For purposes of this comparison, this study will also present the number of seriously injured persons (AIS 3 or greater) and the Malliaris Harm Metric. The costs used for the Fildes Harm metric were normalized to cost of a fatality and are presented in Table 1. As another measure of injury outcome, our analysis also computed the number of serious injuries. Serious injuries are defined to be injuries of AIS level 3 or greater. Like the Harm metric, the use of serious injuries as a metric avoids the biases associated with the use of fatality or total injury counts. Both Harm and number of serious injuries are frequently set as targets for reduction through countermeasure development. COMPARISON OF NEAR AND FAR SIDE IMPACT
The initial step in the analysis was to compare the relative injury risk of near and far side impact. This analysis included only passenger vehicles of model year 1997 or later to capture the effect of the more recently introduced countermeasures likely to be in future fleets. By model year 1997, all U.S. passenger cars were required to meet the dynamic side impact protection provisions of FMVSS No. 214. Likewise, by model year 1997, all passenger cars in the U.S. were required to have both driver and passenger airbags. Similar provisions for dynamic side impact protection and mandatory airbags were required for all light trucks and vans by model year 1999.
Near Side Far Side
Side Struck Occupants (%)AIS 3+ Injured Persons (%)Harm (%)Fatals (%)
Figure 1. Comparison of Near and Far Side Impact Injuries for 3-Point Belted Occupants in Passenger Vehicles of Model Year 1997 and later (NASS/CDS
1997-2002 and FARS 1997-2002)
As illustrated in Figure 1, a side struck occupant has a nearly equal probability of being seated on the near or
far side of the vehicle. Approximately half of the side struck occupants were on the near side, and half were on the far side. Near side impact however carries a significantly higher injury risk. Near side impact resulted in 57% of seriously injured side struck occupants, 70% of the Harm, and 76% of the side impact fatalities Far side impact accounted for 30% of the Harm, 43% of the seriously injured persons, and 24% of the side impact fatalities. The fatality counts for this figure were obtained from the 1997-2002 Fatal Analysis Reporting System (FARS) database. ANALYSIS OF RISK OF INJURY IN FAR SIDE IMPACT
The analysis which follows will focus exclusively on occupants of passenger vehicles subjected to far side impact. The analysis is based upon the NASS/CDS database from 1993-2002. All model years are included. Following the approach described above, only cases with a general area of damage of left or right side are included. Rollovers were excluded. Only 3-point restrained occupants were included in the analysis. As shown in Table 2, these selection criteria resulted in a final sample of over 4500 far side struck occupants. 281 of these occupants were seriously injured. 80 of these occupants were fatally injured. In addition to the unweighted number of cases, the table presents or weighted counts of the number of occupants in each injury severity category. The weighted numbers were developed using the multipliers developed by NASS to permit national estimates of injury. Seriously injured occupants were defined to be occupants with a maximum injury severity of AIS 3 or greater. The fatalities category is a count of occupants whose NASS Treatment variable was fatal. As not all fatally injured occupants die of AIS 6 (unsurvivable) injuries, this approach provides a more accurate count of fatalities than simply counting occupants with a maximum AIS = 6. Fatalities unrelated to the crash were not included in the count of fatalities.
Table 2. Number of Belted Far Side Struck Occupants NASS/CDS 1993-2002
Weighted Unweighted Occupants
Seriously Injured Occupants
Figure 2 presents the distribution of far side impact injuries by body region. The chest was the body region most likely to suffer a serious injury and incurred approximately one-third of all serious injuries (33%).
Head injuries were the second most likely region to suffer a serious injury and accounted for over a quarter of all serious injuries (27%). Head injuries accounted for one-fourth of all Harm, the largest fraction of total Harm. Surprisingly, the upper and lower extremities accounted for a combined one-third of the Harm and one-fourth of the serious injuries. These injuries may be due to the flailing motion of the limbs as the occupant is thrown across the car in a far side impact.
0% 5% 10% 15% 20% 25% 30% 35% 40%
Harm (%)AIS3+ (%)
Figure 2. Distribution of Injuries by Body Region
As shown in Figure 3, drivers accounted for approximately three-fourths of the far side struck occupants as well as three-fourths of the seriously injured occupants and Harm. Right front passengers accounted for approximately 20% of the far side struck occupants, 20% of the Harm, and 25% of the seriously injured occupants. Rear passengers comprised only 7% of the total far side struck occupants and only 3% of the Harm. A test procedure which focuses on the front seat occupants would capture 98% of the seriously injured occupants and 97% of the Harm.
Driver RF Passenger Rear Passenger
Exposed OccupantsMAIS3+ OccupantsHarm
Figure 3. Distribution of Injuries by Occupant Seating Position
A far side impact is much more dangerous for a car occupant than for the occupant of a light truck or van (LTV). Figure 4 presents the distribution of injuries by struck body type. Approximately three-fourths (76%) of the side struck occupants were drivers or passengers of a car. The remaining persons were occupants of an LTV. The LTV category includes pickup trucks, sport
utility vehicles, vans, and minivans. Although car occupants accounted for 76% of side struck persons, car occupants accounted for 83% of the seriously injured persons and 84% of the Harm.
Seriously Inj. Occ(MAIS3+)Harm
Figure 4. Distribution of Injuries by Body Type of Struck Vehicle
Furthermore far side s...